Transducer



- Oct. 28, 1969 SHOOR 3,474,526

TRANSDUCER Filed April 10, 1967 2 Sheets-Sheet 2 INVENTOR.

United States Patent 3,474,526 TRANSDUCER Bernard A. Shoor, San Mateo, Calif., assignor to Endevco Corporation, Pasadena, Calif., a corporation of California Filed Apr. 10, 1967, Ser. No. 629,496 Int. Cl. H041 1/00 US. Cl. 29-595 12 Claims ABSTRACT OF THE DISCLOSURE Stops are provided to limit the displacement of a movmg member of a transducer to microscopic distances of less than about 100 microinches and to an accuracy of microinches or better in order to protect delicate strain sensitive elements against breakage. The gap is formed by bringing hard elements into contact with soft, work hardenable, elements of the assembled unit and then impacting the hard and soft elements together to form the microscopic gaps by virtue of yielding of the soft element and also because of any consequent hardening to resist further yielding.

CROSS REFERENCES TO RELATED APPLICATIONS Patent application Ser. Nos. 377,984, filed June 25, 1964; 421,869, filed Dec. '29, 1964; 599,334, filed Oct. 26, 1966; and 609,553, filed Jan. 16, 1967.

This invention relates to an improved transducer, such as an accelerometer which, in the absence of this invention, could be easily fractured or otherwise damaged when subjected to excessive forces such as those to which the transducer might be exposed in normal use or even to which it might be exposed accidentally as from dropping the transducer a few inches or so onto a stiif or rigid surface. The invention involves providing stops which may be accurately formed to provide gaps of microscopic dimensions.

The invention relates both to the transducer and to the process by which the stops are made. Though applicable to other transducers, the invention will be described with reference to accelerometers, since the invention is especially applicable thereto.

In this patent application, the following abbreviations will be usedfrequently:

( 1) Mil=0.001 inch (2) Microinch or ,u-inch=0.000001 inch (one millionth of an inch) (3) G=acceleration of gravity at the surface of the earth (32.2 ft./sec.

An accelerometer of the type to which this invention has been applied employs an inertial or seismic mass hingedly connected to a base member by means of a flexural connection, hinge, or spring. Delicate piezoresistive elements are supported between the inertial member and the base member across gaps between the two.

In that type of accelerometer, the piezoresistive elements have been of the type described and claimed in copending patent application Ser. No. 599,344 filed Oct. 26, 1966, and patent application Ser. No. 609,553 filed Ian. 16, 1967. Such piezoresistive elements consist of a pair of pads interconnected by a reduced neck having a minimum thickness that may be at little as about 0.5 mil.

In the design of such a transducer various parts of the instrument are so proportioned in dimensions and mass and other physical characteristics that the piezoresistive or other electrosensitive elements are operated over a large fraction, such as about one-third, of the ultimate, or safe, strain to which they can be subjected without damage. Unfortunately, damage, such as breakage, can occur. Often the elements break accidentally because the transducer is dropped just a few inches or about a foot onto a table, such as onto a woodor metal-surfaced table, thereby subjecting the device to an excessive acceleration of many hundreds or thousands of G units at the time of impact. Even placing the accelerometer on a table forcefully by hand can result in breakage. Accelerations of 750 G or more occur so commonly during handling of an accelerometer, either during manufacture or in use, that it becomes a serious problem when one attempts to employ a delicate strain sensitive element such as one of the aforementioned piezoresistive elements as part of a transducer for making measurements of accelerations in a full-scale range .such as 25 G, which is only a small fraction of the acceleration to which the accelerometer may be subjected accidentally or during operation. It is often desirable to provide that an accelerometer which is capable of being used over a specified full-scale range of accelerations, such as 25 G, have a capability of being used over even a larger range without great loss of accuracy in the measurement of acceleration. It is therefore desirable to limit the range of operation of an accelerometer in such a way that the strain sensitive element is always operated within a safe range that exceeds the capability range as well as the fullscale range. The capability range may be about three times the full-scale range and the safe range may be about three times the capability range. The fact that two factors of three are involved is purely coincidental. But in such a case, the full-scale range may be only about 10% of the safe range.

In one line of such accelerometers employing strain sensitive elements of the type mentioned, the elements may be readily employed up to total extension on the element of about 3 n-inches over a full-scale range, with a high degree of linearity. Such linearity is attained for both positive and negative extensions. Greater accelerations of three times full-scale, would produce extensions over an extended range of about 9 ,u-inches. But extensions much larger, above about 27 ,u-inches, whether the extensions be positive or negative, could easily result in breakage or other damage to a large percentage of the accelerometers. Accordingly, it is desirable to provide an arrangement for limiting the action of the accelerometer having a full-scale rating of 25 G to a range of about G. In one such accelerometer which has been employed, the seismic mass is fiexurally mounted at one end for rotation about a hinge axis fixed relative to the base, the free end of the seismic mass moves from its neutral or normal position by a distance of about 20 r-inches when subjected to a full-scale acceleration of 25 G. To provide for over range operation up to a factor of about three without damage, it is therefore desirable to limit the movement of the free end of the seismic mass to about 60 inches. To accomplish this result, it is therefore desirable to provide stops that have gaps which are accurately settable to microscopic distances less than about ,u-inches. This invention makes it possible to set such gaps to an accuracy of :15 u-inches or better. By setting the gaps of an accelerometer to 75 i-inches :15 [L-ll'lChflS, the accelerometer is free to operate over a range of about 3 to 4.5 times full-scale which is well below the safe limit. The invention is also applicable to accelerometers of similar design but operated over other full-scale ranges. For example, it may be employed in accelerometers having a full-scale rating of 1 G, which, therefore, might be damaged when exposed to accelerations greater than 10 G. In this connection, it is to be noted that the range of ex tension of the piezoresistive element remains the same, though the accelerometer is designed to have a full-scale range of only 1 G. For this reason, the gaps must also be set accurately to microscopic distances of the amounts mentioned. Both the structure of the stops employed to attain such accurate setting of microscopic distances and the best process now known for manufacturing such stops is described hereinafter.

In an accelerometer, the seismic mass is generally mounted in a housing which is secured to an object undergoing acceleration. In use, the seismic mass tends to remain stationary in space while the housing moves with the accelerating object. Thus, when considered from an external viewpoint, in an absolute sense, the housing moves; and the seismic mass tends to remain stationary. However, when the seismic mass reaches the limit of its movement relative to the housing, it also moves. Even though the seismic mass tends to remain stationary in space, it is movable relative to the case. Hence, it is sometimes convenient to refer to the seismic mass as the movable member.

The invention is also applicable to other kinds of electromechanical transducers.

In any event, both in accelerometers and in other types of transducers where a physical phenomenon, such as force, acceleration, strain, pressure, temperature, or the like is to be detected, two relatively movable members are involved. Strain sensitive elements, such as piezoresistive or piezoelectric elements are often secured to parts of the relatively movable members of the transducer to detect the relative movement. In order to protect the strain sensitive elements against damage, it is desirable to limit the relative movement of the two movable members or, in other words, the movement of the movable member as viewed from the other, to a range within which the strain sensitive element will not be strained beyond a safe limit. To this end a pair of stop elements are employed to limit the movement of the movable member to microscopic distances in each dirertion along its path of movement.

Prior attempts to guard against damage of accelerometers and other transducers from excessive shocks by limiting the movement of the movable member to such microscopic distances have proved to be ineffective. It is just too difiicult to set a stop with the desired accuracy by conventional methods. Thus, it is practically impossible to set a threaded screw accurately to such a small distance and to maintain it in that position. For example, to set a screw having a fine thread with a pitch of 200 turns per inch to an accuracy of 15 ,u-inches would require setting the screw to within an angle of 1. This fact, taken with others, illustrates the difficulty of using conventional methods of locating stops accurately to form microscopic gaps of less than about 100 ,u-inches with any degree of accuracy.

In accordance with the best mode now known for applying the invention to an accelerometer the movable member, or seismic mass, or inertial mass, is assembled in a housing, or 'case, and stationary stops firmly secured to the case are brought into intimate contact with movable stop elements forming part of or fixed to the movable member, that is the seismic mass. The stop elements are rigidly secured in place while in such contact. One stationary stop element on one side of the movable member and the movable stop element that is directly opposite it, form a pair of stop elements that limit movement of the two members toward each other on that side. In the best embodiment of the invention now known, one of these stop elements is made of a hard material, such as diamond, synthetic sapphire, or tungsten carbide, and the other is made of a work hardenable material such as beryllium copper or certain well-known stainless steel alloys. The terms hard and soft are employed in this application to described the relative hardness of the needle and the pad which together form the stop structure relative to each other.

The hard stop may be in the form of a phonograph needle, while the soft stop may be in the form of a rod with a substantially flat end. After the stationary stop elements are brought into contact with the stop elements and rigidly secured in place, the entire unit is then repeatedly subjected to impact forces in one direction and then the other, along the line of motion of the seismic mass. The impact forces cause the hard stop element to force the material of soft, work hardenable, element to exceed its yield point and thus become permanently deformed. At the same time it becomes work hardened. In a sense, the hard stop acts as a hammer and the soft stop as an anvil. As a result of this process, wells, or dimples are formed in the work hardenable stop elements directly opposite the hard-material stop elements. The wells have a concave bowl-like shape and have upset or protruding annular ridges around their centers. Thus the profiles of the wells are crater-like, being concave in the center and conxex at their edges. By carefully controlling the impacting process, wells of predetermined depth, and hence gaps of predetermined thickness, are formed. In this way, gaps are provided for limiting the range of movement of the movable element to a safe value. By providing such stops on both sides, the transducer is protected against breakage over a wide range of intense shocks to which it may become subjected above the fullscale range over which it is designed to operate.

The invention not only makes it possible to protect the force-sensitive or strain-sensitive elements of the transducer, but also serves to prevent the overload of electrical equipment which is connected to the transducer to detect its output. This avoidance of overloading of the electrical equipment arises because the electrical output of the electrosensitive elements employed in the transducer is limited in accordance with the limitations of the excursion of the movable element by the stops. For this reason, electrical signals generated by these electrosensitive elements are restricted to a limited range for which the electrical equipment may be designed to respond uniformly without being overloaded.

While the invention will be described particularly with reference to accelerometers that employ piezoresistive elements, it will be understood by those skilled in the art that the invention may also be employed in accelerometers that employ other types of eletcrosensitive elements and in other types of transducers.

The foregoing, and other features and advantages of the invention will become apparent from the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is a block diagram of the invention applied to an accelerometer;

FIGS. 1a and 1b are graphs showing how the output of the accelerometer varies with amplitude;

FIG. 2 is a cross-sectional view of the accelerometer taken on the plane 2-2 of FIG. 1;

FIG. 3 is a cross-sectional view taken on the plane 33 of FIG. 2;

FIG. 4 is a perspective view of a piezoresistive element employed in the accelerometer specifically described herein;

FIG. 5 is a schematic drawing illustrating one step in the manufacture of the stops;

FIG. 6 is an exploded perspective view of a subassembly of the accelerometer;

FIG. 7 is a vertical cross-sectional view of the subassembly;

FIGS. 8, 9 and 10 are enlarged detailed drawings employed to explain the process of making the stops; and

FIG. 11 is a vertical cross-sectional view showing an alternative form of the stops.

In the specific embodiment of the invention, an accelerometer 10 is attached to an object 20 undergoing test by means of a stud 30, as illustrated in FIG. 1. The accelerometer develops electrical signals in response to the acceleration of the object 20. These electrical signals are applied to an electrical amplifier 40, the output of which is applied to a recorder 50 or other acceleration indicating or utilization device as indicated in FIG. 1. In many applications, the amplifier is omitted and the amplifier 40 replaced by suitable electrical circuitry for applying the output of the accelerometer to the utilization device 50 without amplification. The accelerometer includes an acceleration detecting unit 100 in the form of an accelerometer cartridge which is rigidly mounted in place on a base 14 which, together with a cap member 16, constitute a sealed housing 19.

The accelerometer unit 10 which detects the acceleration includes a mass structure 120 of cantilever configuration as illustrated in FIGS. 2, 3 and 6, together with other elements mounted on a support tube 130, to form a subassembly or subunit 140 which forms the main part of the accelerometer cartridge 100. This subassembly may be filled with damping fluid such as oil.

The accelerometer is characterized by three principal mutually perpendicular axes. The XX axis is the axis of acceleration which, for purposes of this description, will be considered a vertical axis. The other axes, that is the Y-Y and ZZ axes, lie transverse to the acceleration axis and are considered to lie in a horizontal plane in this description. The YY axis is the longitudinal axis of the mass structure 120 and the axis ZZ is the hinge or rotational axis about which part of the mass structure rotates in response to acceleration of the accelerometer along the axis XX as explained more fully hereinafter.

The base is provided with a threaded mounting hole 21 to receive the stud 30, as indicated in FIG. 1. The base 20 is also provided with an electrical connector 240 through which electrical conductors 210 lead to strain sensitive elements 200 that are mounted in the accelerometer unit. The accelerometer unit is generally of cylindrical configuration and is rigidly mounted in place within the housing 19 by means of a mounting ring 24 and screws 26. The case of the accelerometer unit is in the form of a sealed envelope consisting of three parts: the substantially tubular mounting member 130, an inverted cup-shaped cover 132 secured to one end thereof; and a closure plate 134 at the other end thereof.

The mass structure comprises a unitary member in the form of an enlarged block or inertial member v122 of cylindrical configuration flexurally connected to a base or anchor member 124 by means of a hinge in the form of a reduced portion 123.

The mass structure 120 is formed from a solid block of heavy metal, such as sintered tungsten or other dimensionally stable solid substance of high density. The support tube 130 is constructed of the same heavy metal. By employing a heavy metal for the inertial member, high sensitivity per unit volume is obtained. By employing the same metal for the support tube dilferential thermal expansion of the mass structure and the support tube are avoided, Suitable metals include Densalloy No. 5, manufactured by Powder Alloys Corporation, Clifton, N.J., or similar alloys manufactured by Mallory Metallurgical Company of Indianapolis, Inc. The specific gravity of such material is abnormally high, being about 17. Another suitable material so far as density is concerned, would be depleted uranium which also has abnormally high density.

As indicated in FIG. 6 the mounting tube 130 is provided with a pair of arms 131 that extend horizontally along the longitudinal or YY axis of the mass structure. The base 124 of the mass structure is reduced in thickness in a vertical plane compared with the inertial member and is firmly mounted in place on the mounting tube by being welded or otherwise secured to the ends of the mounting arms 131.

As indicated in FIGS. 6 and 7 a fiexural hinge 123 is formed in the mass structure 120 by means of a pair of vertical slots 125 and 125 that extend horizontally through the neck of the mass structure along the axis XX of rotation and perpendicular to the longitudinal axis YY. The solts extend along a vertical plane perpendicular to the longitudinal axis YY. The hinge axis Z--Z lies at the center of the reduced portion provided by the slots and 125. The neck between the slots is narrow in a vertical plane perpendicular to the axis YY but long along the axis ZZ. For this reason the seismic mass is constrained against movement in any direction other than along a path WW about the horizontal axis ZZ of rotation.

One slot 125 extends upward and the other slot 125 extends downward from the hinge axis ZZ of the block. Each slot 125 and 125 has two opposing faces 126 and 126. The horizontal faces 127 and 127 of the block at the extremeties of each slot are substantially coextensive, when the cantilever mass structure is free from any bending forces, having been part of the same common plane surface prior to forming of the slots.

In eifect, the mass structure comprises two members 122 and 124, namely an inertial member and a base member respectively. The base member 124 is anchored to the case of the cartridge, The inertial member 122 is connected to the base member along a fiexural hinge 123 which is established by the reduced portion between two slots 125 and 125. When installed in the housing the base member remains stationary relative to the case and the inertial member is movable relative to the base when the accelerometer is subjected to acceleration along the axis XX.

Strain sensitive sensors in the form of piezoresistive elements 200 are mounted across the gap on each side of the fiexural member as indicated in FIGS. 2 and 3. The piezoresistive elements are employed to detect changes in the widths of the respective gaps when the inertial member is forced, by virtue of the acceleration of the accelerometer along the axis XX to rotate about the hinge axis ZZ.

Suitable electrical leads 210 connected to the piezoresistive elements 200 lead through the connector 24 to connect the piezoresistive elements in a bridge circuit at the input of the amplifier 40 in conventional manner.

As shown in FIG. 4 each of the piezoresistive elements comprises a unitary member composed of a piezoresistive semiconductor material and has a short narrow neck 202 separating two enlarged coplanar pads 204 and 204 as illustrated in FIG. 4. Each of the piezoresistive elements is mounted across the corresponding slot with the pads cemented with nonconductive cement to the substantially coplanar faces at the ends of the slot. One enlarged pad of each piezoresistive element is cemented to the base 124 and one is cemented to the movable mass 122. The neck 202 of each of the electrosensitive elements bridges the gap at the end of one of the slots. Such electrosensitive elements and their mounting arrangements are described and claimed in more detail in copending patent applications.

The neck 202 of each of the piezoresistive elements is of smooth hourglass configuration. The reduced neck has a length of about 8 mils along a horizontal axis BB parallel to the axis YY, a width of about 7 mils at the thinnest point in the horizontal direction A-A parallel to the axis of rotation ZZ, and a thickness at the thinnest point of about 0.5 mil in a vertical direction CC parallel to the acceleration axis XX. The cross-section is not rectangular but more or less of oval configuration. The hourglass configuration tends to minimize the force necessary to stretch or compress the electrosensitive elements since most of the deformation occurs at the narrowest portion, and the small cross-section allows a relatively small force to deform the piezoresistive elements. Such a piezoresistive element bears thin glass coatings, giving strength to the neck and acting as insulation for the pads.

In the particular embodiment of the invention fullscale deflection is attained when the end of the inertial member is displaced by a small amount of about 20 inches. The piezoresistive elements and the accelerometer are intended to be employed in ranges corresponding to smaller displacements. Thus, for example, the accelerometer in question is designed to produce full-scale output that is very nearly proportioned to acceleration up to a limit of say, 25 G. For convenience, this is referred to as the full-scale range.

When the accelerometer is vibrated or shocked in a direction along the axis X-X, the housing is accelerated in a direction along that axis. This acceleration is communicated to the casing 130 of the accelerometer unit 100 by virtue of the rigid mounting provided by the mounting ring 24 and screw 26. Because of its inertia, the mass member 122 tends to remain stationary during such acceleration, at least at frequencies below the resonant frequency. As a result, the mass member 122 rotates relative to the cartridge casing 130 about the axis ZZ of rotation. This rotation causes the slots 125 to vary in width in a corresponding manner, thus applying compressive and tension forces to the piezoresistive elements 200, thereby straining their neck 202, and causing the electrical resistances between their ends to vary in a manner corresponding to the magnitude of the acceleration. At the same time, the stop gaps P vary in thickness, each increasing in thickness while the other decreases in thickness.

Such piezoresistive elements are easily strained beyond a safe limit, thereby altering their strain vs. resistance characteristics or even breaking them. In practice, the mass structure is so designed that displacement of the inertial member at the extreme end of the range of accelerations to be detected, occurs when the piezoresistive element has been strained to a maximum amount well below the fracture point or to a lesser extent consistent with some design specification such as maintenance of linearity throughout the range of operation. Piezoresistive elements of the kind involved here have a nearly constant gage factor up to about one-tenth of the strain at which they would exceed a safe limit. In that range the change in resistance is very nearly proportional to strain. Above that range departures from linearity occur, the degree of departure increasing with the strain.

In practice, the piezoresistive elements described are safe to use up to a total extension of about 27 ,u-inches. Sometimes, failure occurs earlier for negative extensions than for positive extensions. The extension of 27 ,u-inches corresponds to the extreme value to which to which each of the strain gauge elements may be safely subjected before breaking. Employing a safety factor of about three requires that the maximum extension of the piezo resistive elements be less than about 9 ,u-inches. Taking into account the fact that the mass structure has a length along its longitudinal axis YY about 6 times the distance of the strain gauge elements from the axis of rotation ZZ means that in this case the displacement of the ends of the mass member 122 relative to the cartridge casing 130 should be less than about 54 ,u-inches.

In the particular embodiment of the invention illus trated, the mass structure had a length of about 0.43 inch, and a maximum diameter of 0.190 inch; the base had a height of 90 mils; the fiexural neck had a thickness of about mils in a vertical direction; and the hinge axis ZZ was located about 87 mils from the stationary end of the base. The depths of the slots 125 was 38 mils and the axis WW of the stops was .270 inch from the axis ZZ of rotation, thus establishing the mechanical advantage of 6 thereby, causing changes in thickness of the gaps P to be 6 times changes in extension of the piezoresistive elements, as mentioned above. This particular accelerometer had a resonant frequency of 2500 c.p.s. and by virtue of the use of oil damping could be operated at freequencies from about that value down to zero-frequency, or steady-state, accelerations.

In this invention, special stops are employed to limit the extension of the strain sensitive element to a safe range below which it will not be damaged. In the specific accelerometer described the stops limit the movement of the inertial member of the accelerometer relative to the case, to a safe range below which damage of the strain sensitive elements will not occur when the accelerometer is subject to high accelerations of about 750 G. To this end, special limit stops are located on opposite sides of the inertial member along the path WW of its motion relative to the housing about the rotation axis ZZ. Each of the stops is established by means of a pair of separate stop members that are cemented in the inertial member 122 and the mounting tube 130 respectively.

In the embodiment of the invention disclosed, a stop member 160 in the form of a polished needle, is rgidly mounted in the mounting tube 130 with the needle projecting along the path WW toward the mass member; and an anvil or pad 170 in the form of a cylindrical member is securely mounted in the mass member 122 opposite the needle. The needle 160' is very small having a total length of 45 mils, a shank 163 having a length of about 30 mils, and a tapered or conical end 162 having a length of about 15 mils. The diameter of the shank is about 16 mils and the tip or point of the conical end has a spherical surface having a radius of about 3 mils. The spherical surface of the point is tansgential with the conical surface of the end 162. The pad has a diameter of about 40 mils and a length of about mils,

Each of the pads 170 is formed with a shallow well 173 in its surface opposite the need and the surface is work-shardened to mate with the point 163 of the needle accurately. In the particular embodiment of the invention, the spacing s between the surfaces of each of the opposed stop members 160 and 170 is established between about 75 ,u-inches :15 ,u-inches.

In order to attain accurate setting of the gaps P between the stop members of each pair, radially extending bores 176 are first drilled into diametrically opposite sides of the inertial member. The pads 170 are secured therein by applying epoxy cement to the interior of the bores and pushing the pads into the respective bores, and permitting the cement to harden. The outer ends of the pads are flat and they are flush with the outer surfaces of the inertial member 122.

Additional holes 166 are drilled into the mounting tube in diametrically opposite positions in alignment with the bores 176 in the inertial member 122. Epoxy cement is applied to the side surfaces of the needles, and the needles are then placed in the holes and pushed snugly against the opposite anvil members, and the glue is permitted to harden.

In practice, the mass structure is inserted into the mounting tube and the stationary member thereof. Epoxy cement is applied to the sides of the base 124 of the mass structure and the mass structure is inserted in the mounting tube 130, bringing the arms 131 of the mounting tube into intimate contact with the sides of the cement bearing base 124. This cement is then hardened. In one specific embodiment of the invention, the diameter of the inertial member was .190 inch, the inner diameter of the mounting tube was .200 inch and the outer diameter of the mounting tube was .300 inch. With this arrangement, an annular space having a thickness of about 5 mils Was established between the mass element 122 and the mounting tube 130.

While the deposits of epoxy cement on the needles and pads are still in their fluid state, the needles are pushed into intimate contact with the pads by the application of light pressure. In effect, the needles just barely touch the pads. A very weak U-shaped spring 190 having a pair of inwardly directed fingers are brought into contact with the outer ends of the needles 160 to hold them in place. The spring applies just enough pressure to maintain the needles in a balanced condition in contact with the pads. (In this particular process, a C- clamp 180 as shown in FIG. 5, need not be used).

The entire assembly is then placed in an oven where the epoxy cement is cured and hardened, thereby cementing the mass structure firmly in place within the mounting tube and cementing the pads 170' and needles 160 in place. In this operation, the acceleration axis X-X and the axis WW are maintained in a horizontal position so that the mass structure is located in a central position within the mounting tube. Thereafter, the U- shaped spring 190 is removed. But the needles 160 still bear equally with small predetermined pressures against the outer surfaces of the pads 170.

The mass structure 120, the mounting tube 130, and the stop members 160 and 170 thus arranged and cemented together, form a subassembly which is then processed in the manner described below in order to produce the desired gaps between the needles 160 and the respective pads 170.

To produce microscopic gaps for permitting movement of the inertial member relative to the case of the cartridge up to the predetermined desired limit, the subassembly is mounted on an elevator of a shock table with the axis ZZ of rotation horizontal and it is repeatedly dropped a small distance, such as a few inches or a foot, onto a platform composed of hard rubber or wood or other stiff material. The dropping action is controlled to subject the subassembly to high accelerations which, in the absence of limit stops, could strain the piezoresistive elements beyond the breaking point. For the specific accelerometer illustrated, an acceleration of about 750 G or more are generally applied, applying a somewhat sustained working force to the stop elements. Such shocks force a needle into the corresponding pad, indenting its surface, stressing the material in the pad beyond its yield point, deforming it permanently, and forming a shallow concave bowlshaped well in the surface of the pad. At the same time, the material of the pad is work hardened because of the impacting action. The only heat applied is that generated by the application of the working force.

As such shocks are repeatedly applied, the well, or pocket, is deepened and widened. For this reason and because of the work hardening, each successive shock produces a smaller increase in the gap, the total approaching an ultimate or asymptotic value of about 75 -inches. As the shocks are applied to a pad the material of the pad is forced outwardly from the center producing raised or upset ridges 178. The wells, therefore, have cross-sections of crater-like configuration and their central concave surfaces mate with the points of the needles closely. As a result, a bowl-shaped well, or pocket, is formed in the surface of each pad 170 as illustrated with some exaggeration in FIGS. 9 and 10. The cartridge is then turned over and the process repeated. As a result, a gap having a predetermined microscopic thickness is formed between the stop members of each stop structure.

It will be noted that the tips of the needles are flush with the original surface of the pad, as indicated in FIGS. 9 and 10. The spacing between the tips of the needles is equal to the thickness of the outer surface of the seimic mass prior to the forming process but is less than the spacing between the ridges 178 that protrude from opposite sides of the mass structure.

By impacting the unit in this manner, small gaps of about 75 ,u-inches :about n-inches or less are formed. The mouth of the well is about 0.002 inch wide from crest to crest of ridges on the opposite side thereof. It is to be noted that in this particular case, the gaps provide for an over range capability between about 75 G to about 112 G.

When the gaps are formed by the process described above, the piezoresistive elements may be cemented in place across the ends of the slots 125 either before or after forming the gaps.

When the piezoresistive elements are mounted in place prior to forming the gaps, measurements may be made of maximum changes of resistance of the piezoresistive elements during the impacting process, thus assuring that the piezoresistive elements are not extended beyond their anticipated range of use.

To make such measurements during impacting, the piezoresistive elements are connected in a bridge or other suitable electrical circuit and the output of the circuit is applied to an oscilloscope. From a knowledge of the characteristics of the piezoresistive elements, the current applied to the bridge circuit, and the calibration of the oscilloscope, the ratio of the deflection of the beam of the oscilloscope to the extension of the piezoresistive element is known. Methods of calibrating testing equipment in this manner are readily understood by those skilled in the art and may be readily provided by them without further explanation. In practice, therefore, knowing in advance the oscilloscope beam deflection that would correspond to the extension, eg 9 ,u-inChGS, corresponding to the capability range, the impacting is repeated until such deflection is produced. With the particular accelerometer described in detail herein, about 2 or 3 strong impacts with 750 G acceleration are sufficient to produce gaps of the desired size.

After the gaps have been formed the subassembly is mounted in its cartridge and then assembled in the housing. At some appropriate point in the assembly operation, the accelerometer unit is accurately calibrated on a shaking table or centrifuge and the output checked to determine whether, in fact, the desired capability range of the accelerometer has been attained.

In another method of producing the gaps, after the cement has been applied to the pads and the needles and while the cement is still in a fluid condition, the mounting tube 130 is distorted by being compressed laterally by means of a C-clamp 180 as illustrated in FIG. 5. With parts having the dimensions referred to above, the C- clamp 18!) was compressed to reduce the thickness of the annular space by almost 5 mils directly opposite the C- clamp and to increase the thickness to almost 10 mils along the axis WW through the center of the needles and the pads 170. Again, the needles 160 are pushed into intimate contact with the pads and a very weak U- shaped spring 190 is employed to hold the needles in contact with the pads 170. In this case while the assembly is so clamped, it is placed in an oven and the cement cured while the axes X-X and WW are maintained in their horizontal position. After the cement is cured the C-clamp and U-clamp spring 190 are removed.

Microscopic gaps are then formed by repeatedly im pacting the subassembly under controlled conditions and a predetermined number of times. In the particular case described, three impacts of 750 G are adequate to produce gaps of the desired thickness. In this case the piezoresistive elements are cemented in place after the gaps have been formed and tests later made to determine whether the required capability range has been achieved.

It is important to note that when a C-clamp is employed as described above, the needles project into the pads a distance of approximately 5 mils after the gaps are formed. In this case, the distance between the needles on opposite sides of the seismic mass in the completed structure is not merely less than the distance between the ridges but, in fact, is less than the distance between the surrounding surface of the seismic mass. Thus whether the gaps be formed by the first process described above or by the second process, the spacing between the tips of the needles is less than the spacing between the crests and is no greater than the spacing between the adjacent surface of the seismic mass. By surface of the seismic mass here is meant the surface prior to formation of the gaps. Thus, the needles both extend into said wells when the accelerometer is in a neutral position. And at least one needle or the other extends into its corresponding well under any operating condition.

After the accelerometer has been produced it may be employed to produce electrical signals that accurately correspond in amplitude and wave-shape with the acceleration of an object undergoing tests within the capability range of the accelerometer. But beyond that range the output is clipped. Thus, for example, when the accelerometer is subjected to a sinusoidal acceleration having an amplitude within its range of operation, such as 15 G, the output of the accelerometer and the amplifier 40 is likewise sinusoidal as illustrated in FIG. 1a. But when a sinusoidal acceleration of say 100 G is applied, the peaks of the signals produced by the accelerometer 10 and likewise the peaks of the signal produced by the amplifier 40 are clipped as illustrated in FIG. lb.

In the best embodiment of the invention, the needle is actually in the form of a polished synthetic sapphire phonograph needle. Such needles have the advantage that they are already accurately machined and readily available at low cost on the open market. However, other types of hard materials may be employed, such as diamonds of industrial quality or better, or tungsten carbide. Hard materials having a Mho scale hardness rating of 8 to 10 are very suitable. The work hardenable material may be of any convenient type, such as Series 300 stainless steel, or beryllium copper. Such material is relatively soft, initially having a Rockwell hardness rating of approximately B90. A hardness of B100 to B120 is readily attained as a result of the repeated impacting. The cement used may be commercial grade nonconductive epoxy cement, such as Epoxylite 6203 that is manufactured and sold by Epoxylite Corporation of South El Monte, Calif.

Stops of the type described above may also be formed by reversing the location of the pad and the stylus, that is, by locating the anvils in the mounting tube and locating the styluses in the mass member. The impacting process brings about a shaping of the soft material to fit the harder material establishing a mating relationship between the two which permits ready control of the thickness of the gap and hence limits the motion of the mass member during acceleration.

In another form of the invention, schematically illustrated in FIG. 11, the mass structure 120a is mounted within a support tube 130a with the mass element 122a supported by means of a fiexural hinge 123a from a base member 124a which is cemented directly to the support tube 130a. In this case the seismic mass element 1220 projects from the end of the support tube 130a and the stops are formed by means of a pair of needles 160a that are embedded in and project radially outwardly from the end of the mass element 122a toward blocks 170a that are cemented to the ends of the support member 130a- In practice, the four electrosensitive elements are connected in the arms of a bridge circuit operated with direct current in such a manner as to produce variations of electric potential which is then measured. Alternating current may also be employed. In a practical application, I

the four electrosensitive elements are connected in a bridge circuit to which a direct current is applied across one diagonal, thereby causing an electrical potential corresponding to the displacement to be developed across another diagonal of the bridge curcuit. The output potential is then applied to an amplifier, and the amplified voltage is applied to a utilization device, such as an oscilloscope, an electric meter, or a recording voltmeter, as desired. By virtue of the fact that the motion of the mass is limited, the voltage developed at the output of the bridge is limited in a corresponding manner. In effect, the voltage is clipped. By virtue of this fact, amplifier can be more readily designed taking into account the limits of voltage that are produced at the output of the bridge circuit. By virtue of such limitation of the output of the bridge circuit, the danger of overloading an amplifier and thereby rendering it inoperative during a measurement, may be greatly reduced if not completely eliminated, at least over a wide range of accelerations.

In the foregoing description of the invention, attention has been given to the best method now known for forming the microscopic gaps which serve to permit the accelerometer to work efiectively over a limited range without exceeding the breaking point of its electrosensitive elements. In this process the anvil is automatically shaped to mate with the stylus after the stylus and anvil have been mounted in place. It will be understood, however, that it is possible to attain the desired gap in other ways.

In any event it will be understood that the gaps provided on opposite sides of the mass member are suflicient to enable the mass to move through its full range of operation without bringing either pad in contact with its corresponding opposing stylus and that when the acceleration exceeds some safe limit, the two stop elements are brought into contact, thus stopping further movement of the inertial member and thereby preventing excessive strain from developing in the delicate electrosensitive elements.

The invention may also be practiced by employing stops that are of diiferent shapes than those described and are composed of other materials. Thus, the pad may be composed of hard material and the needle of soft work hardenable material. In this case the points of the needles become blunted during the impacting. Additionally, two work hardenable materials may be employed. In the best embodiment of the invention though, at least one of the stops is composed of work-hardenable material. In the accelerometer described the moveable member is composed of material that is not work hardenable and a work hardenable stop element is fixed on the member in question. All material suitable for use as stops now known have a density less than that of the heavy metals that are most suitable for use as the seismic mass.

Though the invention has been described as being applied to an accelerometer, it may also be applied to other types of transducers for detecting and measuring physical phenomena. Thus, the invention may be applied to any kind of device in which the action of a physical phenomenon produces a displacement which strains an electrosensitive element and the stops may be employed to limit the strain of such electrosensitive elements in order to prevent the strain from exceeding a safe level above which the electrosensitive element might otherwise be broken or otherwise rendered unsuitable for use.

I claim:

1. In a method of establishing a microscopic gap for limiting the relative movement of two mechanical parts of an electromechanical transducer toward each other when subjected to the influence of a force of external origin thereby straining a strain sensitive element in accordance with such relative movement the steps which comprise:

establishing on the respective parts two stop elements at least one of which is composed of a work hardenable material,

bringing said stop elements into contact with each other in the absence of such external force, and

forcing said stop elements together with a working force that exceeds the yield point of said material thereby permanently deforming said one stop member whereby a gap of microscopic dimensions is created between said stop members after said working force has been expended.

2. A method as defined in claim 1 comprising the step of measuring the resultant strain produced in said strain sensitive element produced by the application of said force.

3. A method as defined in claim 2 which comprises again forcing said stop elements together by applying such a working force to them to increase the deformation; and measuring the resultant strain produced in said strain sensitive element by application of the latter working force.

4. A method as defined in claim 2 wherein said measurement is made during the application of said force.

5. In a method of establishing a microscopic gap for limiting the relative movement of two mechanical parts of an electromechanical transducer toward each other when subjected to the influence of a force of external origin thereby straining a strain sensitive element in accordance with such relative movement, the steps which comprise:

selecting a first and second stop member composed respectively of materials that are hard and soft relative to each other,

fixing said stop members to said parts with the stop members in contact with each other in the absence of such external force, and forcing said stop members together with a working force that exceeds the yield point of said soft material thereby permanently deforming said second stop member whereby a gap of microscopic dimensions is created between said stop members after said working force is removed. 6. In the method defined in claim which involves selecting a pointed element and a cylindrical element as said first and second stop members, and causing said pointed member to pierce said cylindrical member during the application of said forming force whereby a well is formed in said cylindrical member to provide such gap.

7. A method as defined in claim 5 in which said working force corresponds to an acceleration of said transducer above a predetermined selected capability range.

8. A method as in claim 5 in which the thickness of the gap is established at such a point that when the two mechanical parts are thereafter moved into contact with each other said strain sensitive element remains free of damage.

9. In a method of establishing two microscopic gaps on opposite sides of a first mechanical member that is movable in opposite directions along a predetermined axis toward two relatively stationary parts of a second mechanical member, the steps which comprise:

establishing two stop elements of said first mechanical member at mutually spaced points on said axis,

locating two stop elements on said parts of said second member in positions opposite said first mentioned stop elements respectively thereby establishing two pairs of opposing stop elements, bringing said latter two stop elements into contact with the first mentioned two stop elements respectively,

afiixing said second mentioned stop elements securely to said parts while in said contacting relation with said first stop elements,

repeatedly forcing one of said mechanical members toward the other to apply to one pair of said stop elements a force that causes the material in at least one of said stop elements to exceed the yield point thereof, thereby permanently deforming said last mentioned one stop element whereby a first gap of microscopic dimensions is created between said one pair of stop elements, and

repeatedly forcing one of said mechanical members toward the other to apply to the remaining pair of said stop elements a force that causes the material in at least one of said last mentioned stop elements to exceed the yield point thereof, thereby permanently deforming said last mentioned stop element whereby a second gap of microscopic dimensions is created between said remaining pair of stop elements.

10. In a method of establishing two microscopic gaps for limiting the relative movement of two mechanical .members as referred to in claim 9 and which form parts of an electromechanical transducer which includes a strain sensitive element that is responsive to relative movement of said mechanical members, the steps which 10 comprise:

measuring the maximum strain produced in said stram element during successive applications of force to each pair of said stop elements.

11. In a method of establishing two microscopic gaps as defined in claim 9 wherein the two stop elements on one of said mechanical members is composed of one material and the two stop elements that are on the other mechanical member are composed of another material and wherein one of said materials is hard and the other material is soft relative to the other material, and wherein the stop elements composed of hard material have a smaller cross-section than the stop elements composed of the soft material, the steps that comprise producing an indentation well in the stop elements formed of soft ma- 5 terial by repeatedly forcing said mechanical members toward each other as described in claim 9.

12. A method for forming a microscopic gap between two stop elements respectively on a pair of relatively movable mechanical parts of an electromechanical transducer that is to be employed over a predetermined capability range and that employs an electrosensitive element to indicate relative movement of said mechanical parts, which element is liable to be damaged when strained beyond a safe limit above said capability range,

the steps that comprise establishing said stop elements on said parts with said stop elements engaging each other, and

while said stop elements are so engaged, subjecting said transducer to an overload greater than such capability range, thereby forcing said stops togrether and deforming at least one of them to provide a gap between them when the overload has been removed with the thickness of the gap of such an amount that when said gap is closed again, said electrosensitive element is strained to an amount less than said safe limit.

References Cited UNITED STATES PATENTS JOHN F. CAMPBELL, Primary Examiner R. B. LAZARUS, Assistant Examiner 60 us. 01. X.R. 

